In some medical cases there is a moral distinction between killing and letting die, but in others there is not. In this paper I present an original and principled account of the moral distinction between killing and letting die. The account provides both an explanation of the moral distinction and an explanation for why the distinction does not always hold. If these explanations are correct, the moral distinction between killing and letting die must be taken seriously in medical contexts. Defeasibly, when an agent kills she takes responsibility, but when an agent lets die she does not take responsibility. Therein lies the moral distinction between killing and letting die. The distinction, however, is defeated when an agent is already responsible for the surrounding situation. In such cases, killing does not involve taking any further responsibility and letting die does not avoid taking any responsibility. Medical examples are frequently complicated because patients' autonomous choices impact upon medical practitioners' surrounding responsibility